The present invention, in some embodiments thereof, relates to synchronization between units in a communication network, and more particularly, but not exclusively, to the issue of synchronization in a broadband over power line (BPL) network.
Power line communications networks typically require modems or communication nodes or communication units that are connected to the electric grid, to provide a network topology based on the electric grid connecting these nodes. Any two nodes in a power line network which are situated within a reasonable distance of each other are able to communicate with each other, but these units must generally have their PHY clocks synchronized so that the units can exchange high modulation transmissions.
According to the current HomePlug AV and BPL Specifications, synchronization is based on a central master unit governing timing over the whole network. The central unit sends beacons or messages to all network nodes to ensure clock synchronization of the nodes' PHY clocks. The result is a commonly used solution called shared clock where one unit shares its clock with the rest of the units. However, this concept was intended for and works best on single hops where all the units hear each other and the central clock more or less directly, and is not optimal for the multi hop environment which is generally the case in a BPL network or a BPL part of a complex of BPL and AV networks. The power line network being used can be quite large and individual units can be connected to numerous neighboring units and those units to numerous others, rendering it impractical to share a single clock source. The result is that PHY level clocks of the neighboring units unable to share a clock source may end up not being fully synchronized.
The shared clock solution teaches the sending of beacons, or synchronization messages between units. Each unit sends a periodic beacon over its area of influence, and these messages also carry information about time domains which are open or closed for transmission. However as the number of units increases, the number of beacons increases exponentially, becoming harder to schedule. The beacons are themselves liable to collision, leading to loss of the corresponding data.
Suitable synchronization between transmitter units and receiving units communicating in a power line network is desirable in order to succeed with high modulations, for example QAM 1024. In fact, the definition of high level, for which good synchronization is needed is basically application dependent. Some applications may generally require good synchronization, so that even 16 QAM can require accuracy of PHY clock synchronization.
Orthogonal Frequency-Division Multiplexing (OFDM) is often used in a power line network since it is particularly suitable for noisy environments. However if each unit uses what is in effect a different clock source, the PHY clocks may be corrupted and consequently loose their synchronization. Without good synchronization, the PHY clocks are not the same, and ppm (parts per million) differences create drifts over time.
Accordingly there is a need for providing an alternative PHY clock synchronization method and system, to provide the necessary high level of PHY clock synchronization, when communicating using OFDM in power line networks.